There's probably an older reference than this, but you may want to
look at
Sander C, Schneider R. Database of homology-derived protein structures and
the structural meaning of sequence alignment. Proteins 1991, 9:56-58.
For a very recent critical reassessment of using sequence identity as
a measure of homology, see
Brenner, SE; Chothia, C; Hubbard, TJP.
Assessing sequence comparison methods with reliable structurally
identified distant evolutionary relationships.
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF
AMERICA, 1998 MAY 26, V95 N11:6073-6078.
--
Ho Leung Ng
holeung at ucla.edu